Since the first cases in the United States were identified in the 1980s, scientists have been divided over that question. Some have suspected that one or more viral infections are likely to play a central role.

But many other researchers — not to mention relatives, friends, employers, doctors and insurers of the million or more Americans estimated to suffer from the illness — have dismissed it as stress-related, psychosomatic or simply imaginary.

Now recent back-to-back announcements have highlighted both the volatility of the issue and the ambiguity of the science, and have alternately heartened and dismayed patients.